The present invention relates to a DC-DC converter for remote power feeding which is connected via a power separating filter to a signal line on which a bit stream signal is superimposed on DC power. In the DC-DC converter an input DC power supplied thereto is switched at a high frequency and applied to the primary winding of a transformer. The power induced in the secondary winding of the transformer is rectified to obtain an output DC voltage different from the input DC voltage.
In such a conventional DC-DC converter, switching on the input side causes abrupt changes in current and voltage, which interact with the stray capacitance of a transformer, the inductances of lead wires and so forth to generate internal common mode noise, that is, a potential difference between the input side and the output side. In an ordinary DC-DC converter, however, the primary and secondary sides of the transformer are grounded to the same potential point, so that internal common mode noise is suppressed. It has been proposed that in the case where the primary and secondary sides of the transformer cannot be grounded in common, the primary and secondary sides are interconnected by a noise suppressing capacitor to reduce the impedance between them.
A remote power feeding DC-DC converter, which is used as a power feeding source or receiving sink in the case of operating a digital line terminating equipment in a subscriber's premises by power feeding from the central office in a digital subscriber line transmission system, is connected via a power separating filter to a signal line on which a bit stream signal is superimposed on DC power. It has been clarified that, in such a case, if the input and output sides of the DC-DC converter are grounded in common, or if a noise suppressing capacitor is connected between the input and output sides, the inductance of the power separating filter and input and output capacitors of the DC-DC converter or the noise suppressing capacitor resonate with each other, by which the common mode rejection ratio of the digital line terminating equipment is reduced and the externally induced common mode noise (i.e. noise with respect to ground potential) is converted into a differential mode noise (i.e. noise induced between lines), which enters into an equalizing amplifier. Accordingly, in the case where an analog telephone line is incorporated in the cable in which the signal line having connected thereto the DC-DC converter is housed, noise resulting from polarity inversion of an exchange of the analog telephone line is induced as the external common mode noise in the signal line connected to the DC-DC converter, and causes a code error. The influence of such external common mode noise could be avoided by making the DC-DC converter float off ground both DC-wise and AC-wise, that is, providing a sufficiently high impedance against the external common mode noise. In this case, however, switching of the DC-DC converter itself produces the common mode noise, causing a code error.